As already discussed, color vision may depend on the presence in the retina of three different types of cones, as well as neurons in the visual pathway that show spectral opposition. Retinal ganglion cells, LGN neurons, and some P cells display spectral opponent properties. The spectral opponent neurons in the striate cortex are found in cortical blobs, and these show double-opponency, in which both the center and the surround portions respond antagonistically
CHAPTER 8 The Special Senses •Fig. 8.12 Simple and Complex Receptive Fields in the Visual Cortex Can Be Generated From Multiple Inputs With Concentric Fields. A andB representon-centerandoff-centerinput,respectively,fromtheretina.Ifthreeon-centercells(A) withadjacentreceptivefieldsconvergedontoonecorticalneuron(E), thatneuron,asimplecell,wouldrespondbesttoalongbarstimulusataspecificlocationandorientation(C). Forthreeoff-centerinputs(B), theresultingreceptivefieldisshowninD. Theconvergenceofmultiplesimplecellsontoanothercorticalneuron(F) wouldresultinacomplexcellthatrespondsbesttoabarstimuluswithaverticalorientationthatcanbeplacedanywherewithinitsreceptivefield.(RedrawnfromSquireLR,etal[eds].Fundamental Neuroscience. SanDiego,CA:AcademicPress;2002.) to two colors. Such a cell, whose center responds to red but not green (R+G−) and whose surround portion responds to green but not red (R−G+), is shown in
Fig. 8.13A . The relationships between the ocular dominance and orientation columns and the cortical color blobs are shown in
Fig. 8.13B
In animal studies, at least 25 different visual areas have been identified in the cerebral cortex, in addition to the striate cortex (Brodmann area 17, or V1). The extrastriate areas include several parallel pathways of visual processing. The P pathway originates with P cells and functions in the recognition of form and color. Structures in the P pathway include LGN layers 3 to 6, layer 4Cβ of the striate cortex, V4 (Brodmann area 19), and several areas in the inferotemporal region (
Fig. 8.14 ). Processing of form includes recognition of complex visual patterns, such as faces. Color information is processed separately from form. The M pathway originates with M cells and functions in motion detection and control of eye movement. Cortical structures in the M pathway include layers 4B and 4Cα of the striate cortex and areas MT (medial temporal) and
I Cortical blobs, concerned with color Stimulus duration Orientation columns I C C •Fig. 8.13 A, Thereceptivefieldandresponsesofadouble-opponent(R+G−/R−G+)neuroninablobofthestriatecortexasitrespondstovariouscombinationsofredandgreenbars.Thebest“on”responseistoaredbarflankedbytwogreenbars.B, Diagramofthecolumnararrangementofthevisualcortex.OculardominancecolumnsareindicatedbyI(foripsilateral)andC(forcontralateral).Orientationcolumnsareindicatedbythesmallercolumnsmarkedwithshortbarsatvaryingangles.ThecorticalblobscontainneuronslikethatinA andhavespectralopponent-receptivefields.
•Fig. 8.14 DistributionofPandMcellinfluencesondifferentareasofthevisualcortex.IT,inferotemporalarea;MST,medialsuperiortemporalarea;MT,medialtemporalarea;V1,striatecortex;V2andV4,higherordervisualareas.MT7 V4 MST Visual cortex Laminae 3-6 Laminae 1,2 Dorsal LGNM-cell pathway P-cell pathway IT V2 V1
MST (medial superior temporal) on the lateral aspect of the temporal lobe, as well as Brodmann area 7a of the parietal lobe (see
Fig. 8.14